Water quality assessment of the Talar River in Mazandaran Province based on a combination of water quality indicators and multivariate modeling

Introduction
The pollutants have exceeded the environmental load capacity of the water resources due to the rapid development and expansion of urban areas, increased industrial production, agricultural activities, and discharging of the domestic sewage into the surface water runoff in recent years. These point and non-point source pollutions have decreased the water quality of the surface water. Water pollution has become a concerning management issue in quality control of the surface water in developing countries. Pollution of urban rivers seriously hinders sustainable economic and social development and threatens human health. Assessment of river water quality according to physical, chemical and biological parameters is the basis of health management of its ecological system. Water quality in any region is affected by two factors: natural processes and human activities such as the transfer of nutrients and heavy metals to surface waters. In this regard, freshwater pollution by heavy metals has become one of the main environmental concerns in recent decades.
Materials and Methods
This study is aimed to evaluate the water quality and the controlling mechanisms through calculation of Drinking Water Quality Index (DWQI) and four pollution indices (PLs). In this regard, the multivariate statistical analysis techniques such as partial least squares regression (PLSR), stepwise multiple linear regression (SMLR) were used, as well as the chemical type of the water has been determined using a three-line piper diagram. Therefore, 26 physicochemical parameters for eight surface water areas of the Talar River were analyzed in this research using standard analytical methods.
Results and Discussion
The results indicate that the surface water resources in this region are determined to be alkaline with Ca-Mg and Ca-Mg-Cl-SO4. According to the results of DWQI analysis, 19% of the total samples were good water, 56% poor and very poor, and 25% were inappropriate for drinking. Also the PI results demonstrate that the surface water is highly affected by Mn and Pb and are under a slight impact of Fe and Cr. The obtained SMLR models of the DWQI and PIs which developed according to the base ions and heavy metals represented the best estimates with R²=1 for QWQI and PIs. According to the Piper diagram, the surface water is affected by the interactions and weathering of rocks. According to Gibbs geochemical diagram, it was shown that surface water points are in areas with weathering and permeability. As a result, these factors have been proposed as the most important factors affecting the geochemistry of surface waters in the study area. The surface waters of the study area are polluted by heavy metals and this pollution increases from the beginning of the river to the station near the Caspian Sea.
Conclusion
The integration of DWQI and PIs is considered as a functional and distinguishing method in the assessment of the surface water quality, and the PLSR and SMLR models can be used to evaluate DWQI and PIs via chemical techniques application. Degradation and reduction of surface water quality can be attributed to the widespread use of agricultural pesticides, industrial activities and poor drainage network. The water quality amount has been severely reduced from upstream to downstream of the Talar River, due to the entry of agricultural effluent and passing through urban and industrial areas. The use of wastewater treatment methods before discharge to freshwater improves water quality and prevents its degradation.